fix(schedule): omit an untouched agent_id from a serialized CronJobPatch

crates/tinyflows-bus/src/schedule/types.rs

@@ -441,7 +441,16 @@ pub struct CronJobPatch {
     /// "clear the agent definition" (`Some(None)`). See
     /// [`deserialize_double_option`] for why the custom deserializer is required
     /// to honor a wire `null` as a clear rather than a silent no-op.
-    #[serde(default, deserialize_with = "deserialize_double_option")]
+    ///
+    /// Omitted when serialized for `None`: the custom deserializer reads a
+    /// present `null` as a clear, so a patch that leaves the agent alone must
+    /// not write `"agent_id": null` or crossing a serialized boundary (the bus)
+    /// would turn every unrelated patch into "clear the agent definition".
+    #[serde(
+        default,
+        deserialize_with = "deserialize_double_option",
+        skip_serializing_if = "Option::is_none"
+    )]
     pub agent_id: Option<Option<String>>,
     /// Set (`Some(Some(_))`) or clear (`Some(None)`) the job's origin
     /// conversation; `None` leaves it unchanged (and is omitted when

crates/tinyflows-bus/src/schedule/wire_tests.rs

@@ -137,3 +137,26 @@ fn job_with_origin_and_run_with_delivery_status_wire_bytes() {
     let run: CronRun = serde_json::from_str(run_json).unwrap();
     assert_eq!(serde_json::to_string(&run).unwrap(), run_json);
 }
+
+/// A patch that does not mention the agent definition must still not mention it
+/// after a trip over the wire; only an explicit clear writes `null`.
+#[test]
+fn an_unrelated_patch_does_not_clear_the_agent_definition_on_a_round_trip() {
+    let untouched = CronJobPatch {
+        enabled: Some(false),
+        ..CronJobPatch::default()
+    };
+    let wire = serde_json::to_value(&untouched).unwrap();
+    assert!(wire.get("agent_id").is_none(), "{wire}");
+    let back: CronJobPatch = serde_json::from_value(wire).unwrap();
+    assert_eq!(back.agent_id, None);
+
+    let cleared = CronJobPatch {
+        agent_id: Some(None),
+        ..CronJobPatch::default()
+    };
+    let wire = serde_json::to_value(&cleared).unwrap();
+    assert_eq!(wire["agent_id"], serde_json::Value::Null);
+    let back: CronJobPatch = serde_json::from_value(wire).unwrap();
+    assert_eq!(back.agent_id, Some(None));
+}